Fire hydrants are in fluid communication with water lines, or a municipal water supply, and they have enough water pressure for the water to rise through the fire hydrant body and spray outwardly when a valve of the fire hydrant is open. Fire hydrants are typically located in public areas making them easy to locate, and easy to access. Unfortunately, the fire hydrant can be opened by an unauthorized person in an attempt to contaminate a public water supply by introducing toxins or other dangerous materials into the fire hydrant outlet, and, thus, into the water supply. A simple flat anti-backflow disc or flap is disclosed in U.S. Pat. No. 7,686,031 which prevents fluid from being pumped into the fire hydrant in an attempt to introduce substances into the water supply. However, the anti-backflow flap is positioned at the base of a fire hydrant tower and inside the fire hydrant tower, and is not practical or useful for retrofitting a fire hydrant outlet extending from the fire hydrant tower. In addition, because the anti-backflow flap is flat it unduly restricts the outflow of water. U.S. Pat. No. 7,240,688 discloses retrofitting a fire hydrant with a secondary valve. However, the secondary valve must be placed in the fire hydrant tower and is not suitable for retrofitting a fire hydrant outlet. U.S. Pat. No. 6,910,495 discloses a backflow prevention system which is positioned inside the outlet of a fire hydrant. This device is not suitable for retrofitting an outlet of a fire hydrant and it unduly restricts the outflow of water. What is needed, but which heretofore has not been available, is a simple backflow prevention insert for retrofitting a fire hydrant outlet which does not restrict outflow.